The modem runs non-free software, which is loaded but not shipped by Replicant.

When using flight mode, The main CPU has to ask the modem to power itself off.

The modem is isolated:

It doesn't use shared memory with the CPU, instead it uses an "HSIC" bus.

We are not aware of it being able to access the GPS, but it woulnd't be surprising if it still could (by having a direct connection to it: since no schematics are publically available we have easy no way to check).